1. Field of the Invention
The present invention relates to a mobile communication system for radio zones each divided into sectors, and more particularly to a mobile communication system having a plurality of base stations positioned respectively in a plurality of radio zones each divided into a plurality of sectors, a plurality of sector antennas positioned respectively in the sectors of each of the radio zones, and a plurality of mobile stations that can be connected to the base stations through radio transmission paths, the mobile communication system being capable of transmitting and receiving a call by way of dynamic channel assignment based on simultaneous use of the same frequency or time slot under conditions free of mutual interference between the base stations.
2. Description of the Prior Art
One conventional mobile communication system for radio zones divided into sectors is disclosed in "Personal Digital Cellular Telecommunication System, RCR Standard--27B" published by Research & Development Center for Radio Systems (RCR). A dynamic channel assignment process which uses a desired-to-interference signal level ratio for assigning channels to incoming and outgoing calls in the conventional mobile communication system will be described below with reference to FIG. 1 of the accompanying drawings.
The term "channel" used below in the dynamic channel allocation process represents a channel as a unit slot. The term "upstream" refers to a direction toward a base station, and the term "downstream" refers to a reverse direction.
Each base station measures upstream interference signal levels in all communication channels using only sector antennas at all times in a step S1. Thereafter, when there is an outgoing/incoming call request from a mobile station in a step S6, the base station measures upstream desired signal levels in all the communication channels using the sector antennas, and calculates upstream desired-to-interference signal level ratios in all the communication channels in a step S2. Then, the base station selects an unused channel with the greatest upstream desired-to-interference signal level ratio as a candidate channel for assignment in a step S8. The base station requests the mobile station to measure a downstream interference signal level in the candidate channel for assignment in a step S18. In response to the request, the mobile station measures the downstream interference average signal level in the candidate channel for assignment in a step S5, and then reports the measured average downstream interference signal level to the base station in a step S20. The base station calculates a downstream desired-to-interference signal level ratio using, as a downstream desired signal level, the received level in its own zone of the outgoing or incoming radio condition report, in a step S11. If the calculated downstream desired-to-interference signal level ratio is equal to or greater than a predetermined signal level ratio threshold in a step S12, then the base station determines the candidate channel for assignment as usable in a step S16. The base station indicates the candidate channel for assignment as a communication channel to the mobile station in a step S13, and starts communicating with the mobile station in a step S15. If the calculated downstream desired-to-interference signal level ratio is smaller than the predetermined signal level ratio threshold in the step S12, then the base station determines the candidate channel for assignment as unusable in a step S17. The base station selects a new candidate channel for assignment in the step S8, and repeats the steps S8 through S12 until a usable candidate channel for assignment is found in the step S16.
The above dynamic channel assignment process in the conventional mobile communication system will be described in greater detail with reference to FIGS. 1 and 2 of the accompanying drawings.
As shown in FIG. 2, the radio zone of a base station 91 is divided into three sectors 95, 96, 97, and the radio zone of a base station 92 is divided into three sectors 98, 99, 100. A mobile station 93 is positioned in the sector 96 for the base station 91, and a mobile station 94 is positioned in the sector 99 for the base station 92. It is now assumed that while the mobile station 93 is communicating with the base station 91, the mobile station 94 sends an outgoing call to the base station 92, which assigns the same channel as the channel that is being used between the mobile station 93 and the base station 91. The sectors 95, 96, 97 are oriented with respect to the base station 91 in the same manner as the sectors 98, 99, 100 are oriented with respect to the base station 92. An upstream interference signal 106 from the mobile station 93 to the base station 92 is attenuated to a large degree because the mobile station 93 is in the opposite direction to the main beam in the sector 99. However, a downstream interference signal 105 from the base station 91 to the mobile station 94 is attenuated to a small degree because the mobile station 94 is in the same direction as the main beam in the sector 96.
In the dynamic channel assignment process shown in FIG. 1, the base station 92 determines a candidate channel for assignment in the step S8 without taking into account a downstream desired(104)-to-interference signal level ratio. Therefore, while an upstream desired(103)-to-interference signal level ratio threshold is being satisfied, a downstream desired(104)-to-interference signal level ratio threshold is not satisfied in the step S17. The number of signals (the steps S18, S20) between the base and mobile stations and the number of times that the level of the downstream interference signal 105 is measured (the step S5) are increased until an assigned channel is determined.
Another disadvantage is that a communication channel cannot quickly be determined as the level of the downstream interference signal 105 is measured by the mobile station 94 in the step S5 after the outgoing/incoming call.